In manufacturing electronic devices such as IC chips, a metallic bump is formed on an electrode of an electronic device for wiring. A bump is formed usually by bonding a ball on the electrode using metallic wire. Such a type of a conventional method for forming a bump is described in the Japanese Patent Kokai No. 64-12555.
In the conventional method for forming a bump described therein, a capillary penetrated by a metallic wire is mounted above a discharge electrode. The distance between the bottom end of the capillary and the top surface of the discharge electrode is set usually from 1 mm to 2 mm, and the distance between the bottom end of the metallic wire and the top surface of the discharge electrode is set from 0.1 mm to 0.5 mm. When a discharge current flows between the metallic wire and the discharge electrode by adding a high voltage such as several thousand volts, the metallic wire gets a high temperature and melts from the bottom end thereof which is near the discharge electrode, then the molten part of the metallic wire becomes spherical and moves up along the metallic wire. Finally, a ball is formed at the bottom end of the metallic wire. The ball and the near part of the remaining metallic wire are recrystallized and coarse particles of crystals are formed. On the other hand, the upper part of the metallic wire is not recrystallized and has fine particles.
Then, the ball is mounted and fixed on an electrode of an electronic device by adding a ultrasonic vibration. The capillary is moved up from the ball to a predetermined position. Then, the metallic fatigue is imposed in the metallic wire to make a crack by adding a ultrasonic vibration. The metallic wire is pulled to be cut at the crack as the capillary moves up. As a result, a bump is formed on the electrode after cutting of the metallic wire.
In the conventional method for forming a bump, however, there is a disadvantage in that the metallic wire can be cut at the interface of recrystallization between the part of coarse particles and that of fine particles, usually 100 .mu.m higher above the ball, where endurance against the shearing stress is smaller than the part where the metallic fatigue is imposed. Consequently, the bump has a wire-shaped part on the main part of the bump. This causes the expansion of the bump horizontally, because the wire-shaped part is crumbled in the process of reshaping of the bump or the inner lead bonding. As a result, the bump can touch the neighboring bump, which causes a short circuit.